In flash memories, it is generally necessary to erase data from an area in which data is stored in order to rewrite the data, and there is therefore an upper limit in the number of data rewrites (that is, the number of erasures). Thus, it may be unable to write new data to a flash memory in which the number of rewrites exceeds the upper limit, and the probability that an error occurs in the written data may increase. The time elapsed until the number of rewrites of a flash memory reaches its upper limit is referred to as “lifetime” of the flash memory. Thus, an FMPK (Flash Memory Package) formed of a plurality of flash memories has also its lifetime.
PTL 1 discloses a technique of dividing a storage area of an FMPK into a write area and a user area of a fixed size, setting an LDEV (Logical Device) which is a plurality of logical storage devices to the user area, and changing the size of a write area allocated to the LDEV according to the frequency of writes to the LDEV.